On a bulletin board beside her desk, Rose Gill Hearn keeps a chart of the dozens of agencies that run the city. The chart is so complex that it resembles an engineering diagram, with lines running from box to box like the electrical wires of a municipal power grid.

As the commissioner of the Department of Investigation, Ms. Hearn is responsible for preventing corruption in a government with 300,000 employees and tens of billions of dollars in annual expenditures. She said the pinup chart helps her keep perspective.

“This is my radar screen,” she said. “This is what I have oversight of.”

Since her appointment in February 2002, Ms. Hearn’s department has pursued corruption among City Council members and tax assessors, deputy commissioners and gas inspectors. Perhaps her highest-profile case is the current investigation of a former police commissioner, Bernard Kerik. Ms. Hearn is working with the Bronx district attorney to determine if Mr. Kerik improperly took gifts and received discounted renovation work from a company that was trying to win city contracts.

The department relies heavily on tips, receiving about 12,000 complaints of corruption a year. Sometimes tips come from city employees who see suspicious behavior among their co-workers, but they also arrive from the companies the city employs.

“‘The city’ is viewed as faceless, and therefore people are desensitized to stealing from it,” she said. “But money stolen or wasted could go to teachers, cops, firefighters.”

Ms. Hearn pursues a wide variety of investigations, from top officials to clerical workers.

Her investigators last week arrested a woman who worked for the Department of Parks and Recreation after she accepted a bribe from an undercover investigator. The sting operation arose from a tip from a parks department employee.

As investigation commissioner, Ms. Hearn is the only head of a city agency who was confirmed by the City Council. Ms. Hearn said she keeps an arms-length relationship with Mayor Bloomberg, who her picked for the job after she served a decade as an assistant U.S. attorney for the Southern District of New York, where she rose to become deputy chief of the criminal division.

When Mr. Bloomberg appointed her, Ms. Hearn recalls that he had just two things to say: “Always do what’s right. And don’t screw it up.”

Ms. Hearn said she has expanded the agency’s efforts to recoup stolen money through lawsuits, forfeitures, and restitution. Since she was appointed, she said the department has recouped about $111 million for the city. In the last year alone, it recovered $22 million.

Ms. Hearn, a New York City native who graduated from Fordham Law School on the Upper West Side, commands a staff of 250 investigators, lawyers, forensic auditors, and computer specialists. Borrowing a tactic from the New York Police Department, she has weekly meetings to go over statistical data and talk about potential citywide problems. When there is a corruption problem at one agency, she looks to see if other agencies have similar problems.

In the last year, a new area of corruption has arisen, to Ms. Hearn’s surprise: corruption involving not-for-profit organizations.

In 2005, the department discovered that employees at the Gloria Wise Boys & Girls Club of America were using money from several dozen city contracts to pay for personal bills, vacations, and new cars. The city has since dropped all contracts with the club.

“It’s a bit of an alarming thing,” Ms. Hearn said. “You don’t expect it from these kinds of organizations.”

While Ms. Hearn focuses on uncovering corruption, she also sees a role for her agency in preventing wrong doing by city workers. “We are not just about slapping handcuffs on people,” Ms. Hearn said. “We will never leave a case … without a policy recommendation.”

Ms. Hearn’s department was created more than 130 years ago, during a time when political corruption was rife. After the conviction of Boss Tweed, the state Legislature armed the department with subpoena power, the ability to take testimony under oath, and statutory independence.

Although there have been many high-profile arrests during her administration, Ms. Hearn said cleaning up the Department of Buildings is the “unsung success story.” For many years, the department typically had double-digit arrests of employees for corruption every other year. Ms. Hearn saturated the department with lectures about corruption and started arresting people who were attempting to bribe workers – a tactic previous administrations had not used. Only three Department of Buildings employees have been arrested in the last two years.

“We’ve changed the culture,” she said.

While Ms. Hearn’s caseload is plentiful, the city rarely sees corruption comparable to the scandals of 100 years ago.

“Things are certainly much better than they were in 1870,” she said.